1. Field of the Invention
The present invention relates to method and apparatus for effecting separation and substantial cancellation of interference at a receiver between a first and a second received digital signal which use the same frequency spectrum and are received from the same general direction and, more particularly, to method and apparatus for effecting separation and substantial cancellation of interference at a receiver between a first and a second digital signal which use the same frequency spectrum and are received from the same general direction in, for example, either overlapping area coverage and spot coverage satellite radiated beams, respectively, or in the same radio beam to achieve increased capacity or service.
2. Description of the Prior Art
In a domestic satellite communication system the coexistence of spot and area coverage beams can be desirable. For example, a separate spot coverage beam can be used for communication between the satellite and each high traffic ground station while an area coverage beam can be used for communication between the satellite and a plurality of low traffic ground stations under conditions where it might not be desirable to interconnect the individual low traffic ground stations to a nearest high traffic ground station for access to the satellite system. To avoid signal degradation and permit separation of the overlapping spot coverage and area coverage beams especially at each spot coverage receiving station, a typical prior art technique would be to use separate bandwidths or polarizations, if possible, for the spot coverage beams and the area coverage beam. Using separate bandwidths, however, results in inefficient use of the frequency spectrum and different polarizations may not be available where dual polarization beams are already used.
Various techniques have been devised to suppress interference between two beams arriving at a receiver from separate directions. In this regard see, for instance, U.S. Pat. Nos. 2,520,184 issued to E. H. Ullrich on Aug. 29, 1950; 3,094,695 issued to D. M. Jahn on June 18, 1963; 3,369,235 issued to C. E. Odams et al on Feb. 13, 1968 and 3,987,444 issued to R. J. Masak et al Oct. 19, 1976. Since the area and spot coverage beams transmitted from a satellite arrive at each spot beam ground station from the same direction, techniques for separating signals from different directions are not usable.
An alternative technique to enable reception of two or more signals at an FM receiver would be to modulate the carrier of each transmitter with a separate address frequency that is assigned to an associated receiver as disclosed, for example, in U.S. Pat. No. Re. 27,478 issued to G. P. Battail et al on Sept. 19, 1972. Such arrangement may be applicable to FM communication systems but does not appear applicable to a digital communication system.
The problem remaining in the prior art is to provide a technique which permits two digital signals using the same frequency spectrum and general transmission direction to be simultaneously transmitted on one radio channel or overlapping spot and area coverage beams with the ability for the signals to be separated at a receiving station intercepting both signals.